Mixers are fundamental building blocks in electronics. A frequency mixer is a 3-port electronic circuit having two input ports and an output port. The mixer multiplies the two incoming electromagnetic signals to output the sum or difference frequency. Radio Frequency (RF) and microwave mixers are commercially available in a large number of styles, packages, and technology platforms ranging from discrete coaxial packages to gallium arsenide (GaAs) integrate circuits (ICs) to fully integrated silicon IC receivers.
Current mixer technology is best classified into two basic categories: hybrid mixers and monolithic mixers. Hybrid mixers are mixers that are manufactured using several different types of materials and manufacturing processes that are combined into a single package. Hybrid mixers are commonly made by attaching discrete semiconductor integrated circuits (ICs) to pre-etched substrates or onto multi-layer low temperature co-fired ceramic (LTCC) substrates. Advantages of hybrid mixers include performance (e.g., broad bandwidth, excellent conversion efficiency) and circuit sophistication. However, a drawback of hybrid mixers is that they are typically large sized, and their method of manufacture is not easily scalable to high volume automated assembly. Instead, hybrid mixers are more suitable for low-to-medium volume applications where a larger size is acceptable in order to achieve the highest available performance.
Monolithic mixers may serve as an alternative to hybrid mixers. With monolithic mixers, the entire mixer is manufactured on a single piece of semiconductor material. Specifically, monolithic mixers, which are built on materials like Silicon (Si), Silicon Germanium (SiGe) or Gallium Arsenide (GaAs), integrate passive mixer circuitry (i.e., baluns, filters networks, couplers, etc.) with IC devices on a single wafer. Monolithic mixers are well suited for high volume applications where size and cost are key factors. Unfortunately, monolithic mixers are inferior to hybrid mixers in terms of their overall performance and in terms of their circuit sophistication. More specifically, monolithic mixers cannot achieve the bandwidth of hybrid mixers because the passive circuits of the monolithic mixer, for example, baluns, are too highly coupled to the ground plane. GaAs wafers, for example, are only about 4 mil thick, implying that the planar monolithic baluns will have a high coupling to chip-ground, which results in a relatively low even mode impedance and a narrow band performance. The effect of the grounded substrate in the monolithic mixer can be partially offset by making the balanced transmission lines very close to each other which effectively lowers the odd mode impedance. However, that approach will force the line widths to shrink resulting in higher insertion loss. For these reasons, it is not practical to make passive monolithic mixers that have bandwidth greater than a few octaves.
Realizing the advantages and drawbacks of hybrid and monolithic mixers, there is a need for new mixer technology that can supply the performance and sophistication of existing hybrid mixers with the small size and highly integrated nature of monolithic mixers.